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CHAPTER 13 STRENGTH UNDER DYNAMIC CONDITIONS Charles R. Mischke, Ph.D., RE. Professor Emeritus of Mechanical Engineering Iowa State University Ames, Iowa 13.1 TESTING METHODS AND PRESENTATION OF RESULTS / 13.3 13.2 SN DIAGRAM FOR SINUSOIDAL AND RANDOM LOADING / 13.7 13.3 FATIGUE-STRENGTH MODIFICATION FACTORS / 13.9 13.4 FLUCTUATING STRESS / 13.24 13.5 COMPLICATED STRESS-VARIATION PATTERNS / 13.29 13.6 STRENGTH AT CRITICAL LOCATIONS / 13.31 13.7 COMBINED LOADING / 13.36 13.8 SURFACE FATIGUE / 13.41 REFERENCES / 13.44 RECOMMENDED READING / 13.45 NOMENCLATURE a Distance, exponent, constant A Area, addition factor, IiN t b Distance, width, exponent B "Li 2 N 1 bhn Brinell hardness, roller or pinion BHN Brinell hardness, cam or gear c Exponent C Coefficient of variation Cp Materials constant in rolling contact d Difference in stress level, diameter d e Equivalent diameter D Damage per cycle or block of cycles D 1 Ideal critical diameter E Young's modulus / Fraction of mean ultimate tensile strength fi Fraction of life measure F Force 3* Significant force in contact fatigue h Depth H 8 Brinell hardness 7 Second area moment k fl , k a Marin surface condition modification factor k b Marin size modification factor k c , k c Marin load modification factor kd, k d Marin temperature modification factor k e , k e Marin miscellaneous-effects modification factor K Load-life constant K/ Fatigue stress concentration factor K, Geometric (theoretical) stress concentration factor € Length log Base 10 logarithm In Natural logarithm L Life measure LN Lognormal m Strain-strengthening exponent, revolutions ratio M Bending moment n, n Design factor N Cycles Nf Cycles to failure TV(JO,, tf) Normal distribution with mean JLI and standard deviation o* p Pressure P Axial load q Notch sensitivity r Notch radius, slope of load line r,- Average peak-to-valley distance R Reliability R a Average deviation from the mean R rms Root-mean-squared deviation from the mean RA Fraction reduction in area RQ As-quenched hardness, Rockwell C scale RT Tempered hardness, Rockwell C scale 5 Strength St 1x Axial endurance limit S' e Rotating-beam endurance limit S f Fatigue strength Sse Torsional endurance limit S U9 S ut Ultimate tensile strength S y Yield strength t f Temperature, 0 F T Torque w Width jc Variable, coordinate y Variable, coordinate z Variable, coordinate, variable of TV(O, z) a Prot loading rate, psi/cycle p Rectangular beam width A Approach of center of roller e True strain £/ True strain at fracture TJ Factor of safety 6 Angle, misalignment angle A, Lognormally distributed |i Mean v Poisson's ratio ^ Normally distributed a Normal stress a fl Normal stress amplitude component G/ Fatigue strength coefficient G m Steady normal stress component G max Largest normal stress c min Smallest normal stress G 0 Nominal normal stress G 0 Strain-strengthening coefficient a Standard deviation T Shear stress (|> Pressure angle <(> Fatigue ratio: <(>&, beading; ^t 0x , axial; <J>,, torsion; <|>o.3o, bending with 0.30- in-diameter rotating specimen O(z) Cumulative distribution function of the standardized normal 13.1 TESTING METHODS AND PRESENTATION OFRESULTS The designer has need of knowledge concerning endurance limit (if one exists) and endurance strengths for materials specified or contemplated. These can be estimated from the following: • Tabulated material properties (experience of others) • Personal or corporate R. R. Moore endurance testing • Uniaxial tension testing and various correlations • For plain carbon steels, if heat treating is involved, Jominy test and estimation of tempering effects by the method of Crafts and Lamont • For low-alloy steels, if heat treating is involved, prediction of the Jominy curve by the method of Grossmann and Fields and estimation of tempering effects by the method of Crafts and Lamont • If less than infinite life is required, estimation from correlations • If cold work or plastic strain is an integral part of the manufacturing process, using the method of Datsko The representation of data gathered in support of fatigue-strength estimation is best made probabilistically, since inferences are being made from the testing of necessar- ily small samples. There is a long history of presentation of these quantities as deter- ministic, which necessitated generous design factors. The plotting of cycles to failure as abscissa and corresponding stress level as ordinate is the common SN curve. When the presentation is made on logarithmic scales, some piecewise rectification may be present, which forms the basis of useful curve fits. Some ferrous materials exhibit a pronounced knee in the curve and then very little dependency of strength with life. Deterministic researchers declared the existence of a zero-slope portion of the curve and coined the name endurance limit for this apparent asymptote. Proba- bilistic methods are not that dogmatic and allow only such statements as, "A null hypothesis of zero slope cannot be rejected at the 0.95 confidence level." Based on many tests over the years, the general form of a steel SN curve is taken to be approximately linear on log-log coordinates in the range 10 3 to 10 6 cycles and nearly invariant beyond 10 7 cycles. With these useful approximations and knowledge that cycles-to-failure distributions at constant stress level are lognormal (cannot be rejected) and that stress-to-failure distributions at constant life are likewise lognor- mal, specialized methods can be used to find some needed attribute of the SN pic- ture. The cost and time penalties associated with developing the complete picture motivate the experimentor to seek only what is needed. 13.1.1 Sparse Survey On the order of a dozen specimens are run to failure in an R. R. Moore apparatus at stress levels giving lives of about 10 3 to 10 7 cycles. The points are plotted on log-log paper, and in the interval 10 3 < N < 10 7 cycles, a "best" straight line is drawn. Those specimens which have not failed by 10 8 or 5 x 10 8 cycles are used as evidence of the existence of an endurance limit. All that this method produces is estimates of two median lines, one of the form Sf = CN b 10 3 <7V<10 6 (13.1) and the other of the form Sf = S' e 7V>10 6 (13.2) This procedure "roughs in" the SN curve as a gross estimate. No standard deviation information is generated, and so no reliability contours may be created. 13.1.2 Constant-Stress-Level Testing If high-cycle fatigue strength in the range of 10 3 to 10 6 cycles is required and reliabil- ity (probability of survival) contours are required, then constant-stress-level testing is useful. A dozen or more specimens are tested at each of several stress levels. These results are plotted on lognormal probability paper to "confirm" by inspection the log- normal distribution, or a statistical goodness-of-fit test (Smirnov-Kolomogorov, chi- squared) is conducted to see if lognormal distribution can be rejected. If not, then reliability contours are established using lognormal statistics. Nothing is learned about endurance limit. Sixty to 100 specimens usually have been expended. 13.1.3 Probit Method If statistical information (mean, standard deviation, distribution) concerning the endurance limit is needed, the probit method is useful. Given a priori knowledge that a "knee" exists, stress levels are selected that at the highest level produce one or two runouts and at the lowest level produce one or two failures. This places the test- ing at the "knee" of the curve and within a couple of standard deviations on either side of the endurance limit. The method requires exploratory testing to estimate the stress levels that will accomplish this. The results of the testing are interpreted as a lognormal distribution of stress either by plotting on probability paper or by using a goodness-of-fit statistical reduction to "confirm" the distribution. If it is confirmed, the mean endurance limit, its variance, and reliability contours can be expressed. The existence of an endurance limit has been assumed, not proven. With specimens declared runouts if they survive to 10 7 cycles, one can be fooled by the "knee" of a nonferrous material which exhibits no endurance limit. 13.1.4 Coaxing It is intuitively appealing to think that more information is given by a failed speci- men than by a censored specimen. In the preceding methods, many of the specimens were unfailed (commonly called runouts). Postulating the existence of an endurance limit and no damage occurring for cycles endured at stress levels less than the endurance limit, a method exists that raises the stress level of unf ailed (by, say, 10 7 cycles) specimens to the next higher stress level and tests to failure starting the cycle count again. Since every specimen fails, the specimen set is smaller. The results are interpreted as a normal stress distribution. The method's assumption that a runout specimen is neither damaged nor strengthened complicates the results, since there is evidence that the endurance limit can be enhanced by such coaxing [13.1]. 13.1.5 Prot Method 1 This method involves steadily increasing the stress level with every cycle. Its advan- tage is reduction in number of specimens; its disadvantage is the introduction of (1) coaxing, (2) an empirical equation, that is, S a = S e ' + Ka" (13.3) f See Ref. [13.2]. where S a = Prot failure stress at loading rate, a psi/cycle Sg = material endurance limit K,n= material constants a = loading rate, psi/cycle and (3) an extrapolation procedure. More detail is available in Collins [13.3]. 13.1.6 Up-Down Method 1 The up-down method of testing is a common scheme for reducing R. R. Moore data to an estimate of the endurance limit. It is adaptable to seeking endurance strength at any arbitrary number of cycles. Figure 13.1 shows the data from 54 specimens FIGURE 13.1 An up-down fatigue test conducted on 54 specimens. (From Ransom [13.5], with permission.) gathered for determining the endurance strength at 10 7 cycles. The step size was 0.5 kpsi.The first specimen at a stress level of 46.5 kpsi failed before reaching 10 7 cycles, and so the next lower stress level of 46.0 kpsi was used on the subsequent specimen. It also failed before 10 7 cycles. The third specimen, at 45.5 kpsi, survived 10 7 cycles, and so the stress level was increased. The data-reduction procedure eliminates spec- imens until the first runout-fail pair is encountered. We eliminate the first specimen and add as an observation the next (no. 55) specimen, a = 46.5 kpsi.The second step is to identify the least-frequent event—failures or runouts. Since there are 27 failures and 27 runouts, we arbitrarily choose failures and tabulate N 1 , iN t , and PN 1 as shown in Table 13.1. We define A = ZiN 1 and B = Zi 2 N 1 . The estimate of the mean of the 10 7 - cycle strength is * =s ° +d (w^} < 13 - 4 > where S 0 = the lowest stress level on which the less frequent event occurs, d = the stress-level increment or step, and N 1 = the number of less frequent events at stress level a/. Use + 1 A if the less frequent event is runout and - 1 A if it is failure. The estimate of the mean 10 7 -cycle strength is f See Refs. [13.4] and [13.5]. TABLE 13.1 Extension of Up-Down Fatigue Data Class failures Stress level, Coded I \ kpsi level N t IN 1 / 2 AT, 48.5 7 1 7 49 48.0 6 4 24 144 47.5 5 1 5 25 47.0 4 3 12 48 46.5 3 5 15 45 46.0 2 8 16 32 45.5 1 333 45.0 O _2 J) J) Z 27 82 346 A - 45.0 + 0.5 (U - y) = 46.27 kpsi The standard deviation is [ FiYN — A 2 1 OWV +0-029 (13.5) (±l\i) J as long as (BZN 1 - A 2 ^(ZN 1 ) 2 > 0.3. Substituting test data into Eq. (13.5) gives or = 1.620(0.5) [ 342 C 2 ^" 822 + 0 .029J = 2.93 kpsi The result of the up-down test can be expressed as (Sf )io 7 (jl, tf ) or (S/) 10 7(46.27,2.93). Consult Refs. [13.3] and [13.4] for modification of the usual f-statistic method of placing a confidence interval on ji and Ref. [13.4] for placing a confidence interval on tf. A point estimate of the coefficient of variation is oYji = 2.93/46.27, or 0.063. Coef- ficients of variation larger than 0.1 have been observed in steels. One must examine the sources of tables that display a single value for an endurance strength to discover whether the mean or the smallest value in a sample is being reported. This can also reveal the coefficient of variation. This is still not enough information upon which a designer can act. 13.2 SNDIAGRAMFORSINUSOIDAL AND RANDOM LOADING The usual presentation of R. R. Moore testing results is on a plot of Sf (or S//S M ) ver- sus N, commonly on log-log coordinates because segments of the locus appear to be rectified. Figure 13.2 is a common example. Because of the dispersion in results, sometimes a ±3tf band is placed about the median locus or (preferably) the data points are shown as in Fig. 13.3. In any presentation of this sort, the only things that might be true are the observations. All lines or loci are curve fits of convenience, there being no theory to suggest a rational form. What will endure is the data and not CYCLES TO FAILURE FIGURE 13.2 Fatigue data on 2024-T3 aluminum alloy for narrow-band random loading, A, and for constant-amplitude loading, O. (Adapted with permission from Haugen [13.14], p. 339.) CYCLES FIGURE 13.3 Statistical SN diagram for constant-amplitude and narrow-band random loading for a low-alloy steel. Note the absence of a "knee" in the random loading. the loci. Unfortunately, too much reported work is presented without data; hence early effort is largely lost as we learn more. The R. R. Moore test is a sinusoidal completely reversed flexural loading, which is typical of much rotating machinery, but not of other forms of fatigue. Narrow-band random loading (zero mean) exhibits a lower strength than constant-amplitude sine- wave loading. Figure 13.3 is an example of a distributional presentation, and Fig. 13.2 shows the difference between sinusoidal and random-loading strength. 13.3 FATIGUE-STRENGTH MODIFICATION FACTORS The results of endurance testing are often made available to the designer in a con- cise form by metals suppliers and company materials sections. Plotting coordinates are chosen so that it is just as easy to enter the plot with maximum-stress, minimum- stress information as steady and alternating stresses. The failure contours are indexed from about 10 3 cycles up to about 10 9 cycles. Figures 13.4,13.5, and 13.6 are HGURE 13.4 Fatigue-strength diagram for 2024-T3,2024-T4, and 2014-T6 aluminum alloys, axial loading. Average of test data for polished specimens (unclad) from rolled and drawn sheet and bar. Static properties for 2024: S u = 72 kpsi, S y = 52 kpsi; for 2014: S 11 = 72 kpsi, S y = 63 kpsi. (Grumman Aerospace Corp.) examples. The usual testing basis is bending fatigue, zero mean, constant amplitude. Figure 13.6 represents axial fatigue. The problem for the designer is how to adjust this information to account for all the discrepancies between the R. R. Moore spec- imen and the contemplated machine part. The Marin approach [13.6] is to introduce multiplicative modification factors to adjust the endurance limit, in the determinis- tic form S e = k a k b k c k d k e St (13.6) MINIMUM STRESS O 1n , kpsi MAXIMUM STRESS FIGURE 13.5 Fatigue-strength diagram for alloy steel, S u = 125 to 180 kpsi, axial loading. Aver- age of test data for polished specimens of AISI 4340 steel (also applicable to other alloy steels, such as AISI 2330,4130,8630). (Grumman Aerospace Corp.) FIGURE 13.6 Fatigue-strength diagram for 7075-T6 aluminum alloy, axial loading. Average of test data for polished specimens (unclad) from rolled and drawn sheet and bar. Static prop- erties: S u = 82 kpsi, S^ = 75 kpsi. (Grumman Aerospace Corp.) MAXIMUM STRESS a , % of S ma X U MAXIMUM STRESS a , kpsi MINIMUM STRESS MINIMUM STRESS